In recent years, as the demand for an LED (Light Emitting Diode) applied to lighting is increasing, the development of LED having higher luminance is proceeding.
A high-luminance LED emits a light with high intensity, and further emits a large quantity of heat due to a large current flowing the LED. Therefore, the members constituting the LED are exposed to a light with high intensity and placed under high temperature conditions for a long term. For this reason, the members constituting the LED are required to individually have high light resistance and high heat resistance.
There are generally representative white light emitting apparatuses (hereinafter, frequently referred to as “white LEDs”) of the following three types (Non-Patent Literature 1).
The white LED of the first type has only the light emitting devices in the one package therefor and does not need a binder, such as an epoxy resin, and therefore can achieve high light resistance and high heat resistance. The white LED of the first type, however, has a problem in that it is difficult to control the luminances and color tones of the light emitting devices of three colors, so that the construction of the circuit for the control is inevitably complicated, increasing the cost for production of the LED.
A white LED of the second type has in one package for the LED an ultraviolet (hereinafter, frequently referred to as “UV”) light emitting device and a binder, such as an epoxy resin, covering the ultraviolet light emitting device, wherein the binder has dispersed therein an ultraviolet photoexcited red fluorescent substance, an ultraviolet photoexcited green fluorescent substance, and an ultraviolet photoexcited blue fluorescent substance.
A white LED of the third type has in one package for the LED a blue light emitting device and a binder, such as an epoxy resin, covering the blue light emitting device, wherein the binder has dispersed therein a blue-photoexcited red fluorescent substance and a blue-photoexcited green fluorescent substance.
In each of the white LEDs of the second and third types, only one light emitting device is used in the one package. Therefore, in the white LEDs of the second and third types, the luminance and color tone are easily controlled, as compared to those in the white LED of the first type, and hence the circuit for the control can be simplified. Thus, the white LEDs of the second and third types have an advantage in that the cost for production of the LED can be reduced. Further, the white LEDs of the second and third types also have an advantage in that it is possible to increase the width in which the color temperature is controlled.
However, both the white LEDs of the second and third types use a binder, and therefore pose a problem in that the binder is exposed to a light with high intensity and placed under high temperature conditions for a long term and hence is likely to deteriorate, so that the binder suffers discoloration to lower the light transmittance, reducing the light emission efficiency.
Moreover, when a large current flows through the light emitting device to cause light emission with high luminance, not only the deterioration of the binder but also a lowering of the properties of the fluorescent substances may occur Non-Patent Literature 2).
The white LED is not limited to one having the above-mentioned construction using the lights of three colors, but may have a construction using lights of two colors having the relationship of complementary colors and passing through the CIE chromaticity coordinates (0.33, 0.33) for a white light. For example, there is a white LED having in one package for the LED a combination of a blue light emitting device and a blue-photoexcited yellow fluorescent substance in a particulate form dispersed in a binder, such as an epoxy resin (Patent Literature 1). However, the white LED having such a construction using the lights of two colors still has a problem of the lowering of the light emission efficiency due to the deterioration of the binder.
For solving the problem of the deterioration of the binder, an attempt to use a silicone resin as a binder instead of the epoxy resin has been made. However, the problem has not completely been solved.